The present invention relates to a method for the recovery of lead from a lead-bearing sulfide concentrate by heating the concentrate so that the compounds of lead pass into the gas phase.
Most of the world's lead is produced from lead-bearing sulfide concentrates by a sintering-shaft-furnace process. In the sintering machine the concentrate is oxidized in order to remove the sulfur and it is brought into a particle form suitable for shaft-furnace reduction.
The greatest disadvantage of the process is its large quantities of waste gas, which are produced during both the sintering and the shaft-furnace process. It has been estimated that process and ventilation gases which contain sulfur dioxide and dusts are produced at a rate of about 670 kmol (15 000 Nm.sup.3) per one tonne of concentrate. The purification of the waste gases to correspond to the current requirements of environmental protection causes a considerable increase in the costs of lead production.
The aim of recent research has been to create a process in which the sulfur dioxide is obtained in a concentrated form and the quantity of dust-bearing waste gases is minimal. In principle, a single-stage process is possible for pure concentrates which contain only very little quarz. Sulfidic lead concentrate is oxidized directly to metal in one process stage. As a sub-reaction, lead sulfide sulfidizes first to oxide according to the reaction below EQU PbS+1/2O.sub.2 =PbO+SO.sub.2
Thereafter, the excess lead sulfide reduces the oxide to metal according to the following reaction EQU 2PbO+PbS=3Pb+SO.sub.2
At a low operating temperature of the process, lead sulfate and oxysulfates are obtained instead of oxide. Metallic lead is produced when these compounds react with lead sulfide.
The single-stage lead production process is best applicable to pure concentrates. Owing to the great mutual affinity of lead oxide and silica, the concentration of lead in the slag increases and the yield of metallic lead decreases as the concentration of quartz in the concentrate increases. Releasing lead from the silicate requires so low an oxygen pressure that, in the presence of sulfur dioxide, lead sulfide is obtained instead of metallic lead.
At those temperatures and oxygen pressures which are used in direct production of lead, the zinc present in the concentrate oxidizes and passes into the slag. In order to maintain the melting point of the slag sufficiently low, the slag has to be fluxed, which for its part increases the losses of lead into the slag.
Processes of several stages have been applied to the treatment of impure concentrates. It has been possible to eliminate the disadvantages of the sintering process, i.e. dilute sulfur dioxide gas and passing of lead oxide dust into the environment, the formation of sulfates and difficulties in temperature control, by shifting to closed reactors, the product of which is a melt containing the lead oxide. Such is, for example, the Kivcet process (FI Lay-Open Print 56028).
The vapor pressure of lead sulfide especially, but also of lead oxide, is high at the operating temperatures of the lead production process. This is the reason for the large quantities of fly dusts, which are typical of the process and highly detrimental. Both in a multi-stage and in a single-stage process there occurs volatilization of both lead sulfide and oxide. The boiling point of lead sulfide is about 1610 K. and that of lead oxide about 1810 K., and so at the processing temperatures the gas may contain large quantities of the said compounds. Volatilized lead compounds leave the processing apparatus along with the sulfur-dioxide bearing gas.
Depending on the sulfur dioxide pressure, only lead sulfide, sulfate and various oxysulfates are stable below 1050-1150 K. For this reason, the dust separated from cooled gas, the dust possibly representing a very high proportion of the lead amount fed into the process, mainly consists of these compounds. The amount of lead oxide is less.
Feeding the fly dust to the oxide reduction stage is not possible because of its sulfur content. During the reduction stage the sulfur would be reduced and would leave along with the gas in the form of lead sulfide. Likewise, the concentration of sulfur in the lead produced would be high. The most common method of treating the dust is to feed it back to the oxidation stage together with fresh concentrate. However, there is the disadvantage in the amount of energy required by the endothermal decomposition reactions of the sulfates and the increase in the gas quantity in the process owing to the high rate of recycling of dust.
One of the main objectives in the development of lead processes has been to reduce the amounts of dust. One method to achieve this has been to cool the gas in the outlet section of the oxidation reactor so that the lead compounds condense and fall back into the hot melt. This procedure is used in the Kivcet process. However, the return of cooled dust, which possibly contains sulfates, results in excessive consumption of heat.
Furthermore, U.S. Pat. No. 4,169,725 discloses a process for the suspension smelting of sulfidic complex or mixed ores or concentrates in order to separate the impurities present in them, a process in which the non-volatile impurities are subjected to a reducing or sulfidizing treatment in the lower section of the reaction zone in order to return them to the gas phase before the solid is separated and impinges against the melt. By this procedure it is ensured that the impurities will not substantially pass into the melt but remain in the gas phase. In this patent it is noted that a considerable amount of lead oxide can be made to remain in the gas phase in spite of the reduction and that the lead sulfides can be evaporated to low concentrations in the gas phase without the oxidation of the sulfide. It is also noted that the reduction and sulfidization of lead from molten silicates is difficult.
Another method, applied in several processes, for decreasing the amount of dust is to inject the sulfide concentrate either to the melt surface or below the surface of the melt in the furnace. Thus a rapid dissolving of the sulfide in the molten lead or a reaction with the lead oxide present in the slag is effected, and thereby the activity of the lead sulfide decreases and its volatilization decreases.
None of the methods described above entirely eliminates the dust problem involved in lead production processes. A large proportion of the lead content of the concentrate continues to be removed along with the gas and is sulfated or sulfidized during the cooling of the gas.
The object of the present invention is thus to eliminate the entire dust problem involved in prior known lead production processes and to provide a process for the recovery of lead from a lead-bearing sulfide concentrate by heating the concentrate in such a manner that the compounds of lead pass into the gas phase.